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How to Create Lists of Objects and Append Items to Lists in Classes in Python

This guide explores how to create lists of objects and manage lists within Python classes. We'll cover instantiation, appending to lists within the class and using class variables vs instance variables when working with lists. You'll learn practical techniques for creating and managing collections of objects in your Python code.

Creating a List of Objects​

You can create a list of objects by instantiating a class in a loop and appending the results to a list.

1. Using a for Loop​

The most straightforward approach is to use a for loop with the range() function to create a sequence and then instantiate the class and append the result to the list:

class Employee():
    def __init__(self, id):
        self.id = id

list_of_objects = []
for i in range(5):
    list_of_objects.append(Employee(i))

print(list_of_objects)
for obj in list_of_objects:
    print(obj.id)

Output

[<__main__.Employee object at 0x7f46887a4f10>, <__main__.Employee object at 0x7f46887a4fa0>, <__main__.Employee object at 0x7f46887d0430>, <__main__.Employee object at 0x7f46887380a0>, <__main__.Employee object at 0x7f4688738250>]
0
1
2
3
4
  • The for loop iterates five times, creating five Employee objects, appending the object to list_of_objects in every iteration.
  • Then the code loops over the list of objects and prints the id of each object.
note

To control how the object is printed, it's important to implement the __repr__ method in your class. This ensures that the objects will be displayed in a clear, user-friendly format.

    class Employee():
        def __init__(self, id):
            self.id = id

        def __repr__(self):
            return str(self.id)
    list_of_objects = []
    for i in range(5):
        list_of_objects.append(Employee(i))
    
    print(list_of_objects) # Output: [0, 1, 2, 3, 4]

Also, you can dynamically set attributes to the object using the setattr() function:

class Employee():
    def __init__(self, id):
        self.id = id

    def __repr__(self):
        return str(self.id)


list_of_objects = []

for i in range(3):
    obj = Employee(i)

    setattr(obj, 'topic', 'Python')
    setattr(obj, 'salary', 100)

    list_of_objects.append(obj)

print(list_of_objects)

for obj in list_of_objects:
    print(getattr(obj, 'topic'))
    print(getattr(obj, 'salary'))

Output:

[0, 1, 2]
Python
100
Python
100
Python
100
  • The setattr() function can be used to add new or modify existing attributes dynamically to an object.

2. Using a List Comprehension​

You can also use a list comprehension to create a list of objects in a more concise manner:

class Employee():
    def __init__(self, id):
        self.id = id
    def __repr__(self):
        return str(self.id)
list_of_objects = [Employee(i) for i in range(1, 6)]
print(list_of_objects) # Output: [1, 2, 3, 4, 5]
for obj in list_of_objects:
    print(obj.id)

Output:

1
2
3
4
5
  • The list comprehension [Employee(i) for i in range(1, 6)] creates a list where each element is the result of instantiating the Employee class.

Appending Items to a List in a Class​

When you need a list associated with your class, you can initialize it and manage it in the class itself.

1. Using Instance Variables​

Instance variables are unique to each instance of a class. This means, each object will have it's own independent copy of the list. Use the append() method to add single items to the list.

class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary
        self.tasks = []  # Instance variable initialized as an empty list

    def add_task(self, task):
        self.tasks.append(task)
        return self.tasks

tom = Employee('Tom', 100)
tom.add_task('develop')
tom.add_task('ship')
print(tom.tasks) # Output: ['develop', 'ship']

alice = Employee('Alice', 1000)
alice.add_task('design')
alice.add_task('test')
print(alice.tasks) # Output: ['design', 'test']

Output:

['develop', 'ship']
['design', 'test']
  • In the __init__() method, the instance variable self.tasks is initialized to an empty list.
  • The add_task() method uses the append() method to add new items to the self.tasks list, and returns the list.
  • Each employee object will have it's own, independent list of tasks.

2. Using Class Variables​

Class variables are shared by all instances of the class. Use class methods to append items to class variables.

class Employee():
    tasks = [] # Class variable

    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    @classmethod
    def add_task(cls, task):
        cls.tasks.append(task)
        return cls.tasks
Employee.add_task('develop')
Employee.add_task('ship')
print(Employee.tasks)

alice = Employee('Alice', 1000)
print(alice.tasks)

tom = Employee('Tom', 100)
print(tom.tasks)

Output:

['develop', 'ship']
['develop', 'ship']
['develop', 'ship']
  • Class variable tasks is declared on the class, not on a specific object.
  • Class methods are created using the @classmethod decorator. The first argument in the method is cls which refers to the class.
  • When class method add_task() is called, it appends the tasks to the tasks variable which is a list on the class.
  • All of the instances will refer to the same tasks list which is on the class.

Appending Multiple Items with extend()​

To append multiple items from an iterable to a list, use the extend() method in the class:

class Employee():
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary
        self.tasks = []  # initialize list

    def add_tasks(self, iterable_of_tasks):
        self.tasks.extend(iterable_of_tasks)
        return self.tasks

tom = Employee('TomNolan', 100)
tom.add_tasks(['develop', 'test', 'ship'])
print(tom.tasks)  # Output: ['develop', 'test', 'ship']
  • The extend() method appends all items from the given iterable to the list.

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